This invention relates to rotary transmissions or clutches of the viscous fluid type, comprising an outer rotary casing with an inner rotary element, having closely spaced surfaces between which the viscous fluid can generate torque in the well-known manner.
The invention is particularly applicable to transmissions of the kind in which it is required to reduce or disconnect the drive at intervals and the invention is particularly, though not exclusively, applicable to an automatically controlled transmission or drive between an internal combustion engine on a vehicle and an auxiliary component such as a cooling fan. In the case of cooling fans in particular it is desirable, particularly in heavy motor vehicles where the cooling fan absorbs considerable power, to be able to disconnect the drive when cooling is not needed. This can be detected, for example, by a thermal sensor in the coolant circuit, or exposed to the air stream passing through a radiator, or some other component of the engine.
There are many existing viscous fluid transmissions having means for disconnecting the drive but they tend to be complicated and expensive to produce and for various reasons their operating life is limited. It is common practice to provide a pump which continuously withdraws viscous fluid from an operating zone in which the fluid provides viscous trnsmission of torque to a reservoir where the fluid is no longer effective and the drive is interrupted. In order to control the operation of the drive it has been proposed to incorporate valves operated externally for controlling the flow of fluid towards or away from the reservoir. It has also been proposed to control the quantity of fluid in the operating zone by means of an internal displaceable wall or membrane. As stated above, however, all existing systems suffer from said disadvantages and it is an object of the invention to provide an improved transmission of this general type which will be simple to manufacture and reliable in operation.